1. Field of the Invention
The present invention relates generally to a bolt and a nut. More particularly, the present invention relates to a high-strength bolt and a high-strength nut capable of stably providing a desired tightening torque, when used with a standardized nut and a standardized bolt, respectively.
2. Description of Related Art
A bolt and nut set is in wide practical use today for clamping two or more elements. So ensuring a desired clamping force, it is particularly important to make the tightening torque stable. The tightening torque T is generally represented as a function of the nominal diameter d of bolts and, theoretically, so long as the bolts have the same nominal diameter d, the same tightening operation provides substantially the same tightening torque, thus ensuring a desired clamping force.
in practice, however, such a desired clamping force cannot be stably obtained. The reason for this is that bolts and nuts are generally manufactured by forging, rolling, tapping, or the like and, hence, the flanks of thread ridges are not smooth flat planes, resulting in variations in contact pressure between the flanks of the thread ridges of the bolt and those of the thread ridges of the nut (see FIG. 3B depicting the load distribution F2). In general, the contact of the bolt and the nut under pressure with elements to be clamped thereby is followed by contraction of the nut and expansion of the shank extending between the bolt head and the nut. At this moment, the thread ridges of the bolt and those of the nut tend to deform in opposite directions, and the load acting on the thread ridges differs in a direction axially of the bolt. As a result, even a small unintentional error during the tightening operation occasionally causes a variation in the tightening torque, which in turn causes a somewhat variable axial tension of the bolt. For this reason, the actual circumstances are such that commercially available bolts and nuts cannot provide the desired clamping force.
To overcome the above-described problem, a high-strength bolt of a type being sheared by torque has hitherto been proposed having a grid portion formed at a free end of the shank thereof and a notched neck, which is sheared by a predetermined tightening torque acting on the grip portion.
However, because this kind of high-strength bolt requires processing to form the notched neck in the shank thereof, not only the manufacture thereof is very troublesome, but also the tightening torque greatly depends upon the processing accuracy of the notched neck. Accordingly, the manufacture of this bolt requires highly accurate processing. Also, the problem remains in that when the bolt is coupled with a nut for clamping, partial variations contact pressure between the flanks of the thread ridges of the former and those of the thread ridges of the latter make the axial tension of the bolt variable and the clamping force unstable.